3rd party URL shortening services inevitably make links more fragile and vulnerable to a single point of failure due to maintenance or being hacked. The only work around for this problem is for sites to host their own URL shorteners, as http://flickr.com has done with http://flic.kr/ URLs.
Typical URL shorteners take an arbitrary URL and return a short URL, usually on a shorter domain, followed by a seemingly random set of alphanumeric digits that usually indicate an id of an entry in a database which has the original URL.
Thus to develop my own URL shortener I realized first I needed to come up with a good way to maximally compress a number (or a few) into characters in a URL.
While many existing shorteners use numbers and letters, I found a few flaws and vulnerabilities in their choice of what set of numbers and letters to use and decided to use a methodology based on usability and robustness to deliberately choose which characters to use.
